This invention relates to an improved alloy flow system for use in the pressure casting of alloys.
In a number of recent patent applications, we have disclosed inventions relating to the pressure casting of alloys, utilising what is referred to as a controlled expansion port (or CEP). Those applications include PCT/AU98/00987, relating to magnesium alloy pressure casting and PCT/01/01058, relating to aluminium alloy pressure casting. They also include the further applications PCT/AU01/00595 and PCT/AU01/01290, as well as Australian provisional applications PR7214, PR7215, PR7216, PR7217 and PR7218 each filed on 23 Aug. 2001. These further applications relate variously to the pressure casting of magnesium, aluminium and other pressure castable alloys and to devices and apparatus for use in pressure casting of those alloys.
As indicated, a CEP is utilised in the inventions of the above-identified patent applications. A CEP is a relatively short part of the alloy flow path which increases in cross-sectional area, from an inlet end to an outlet end of the CEP, such that alloy flowing through the CEP has a substantially lower flow velocity at its outlet end relative to the inlet end. The reduction in flow velocity is such that, in its flow through the CEP, the alloy undergoes a change in its state. That is, with molten alloy received from a pressurised source of supply to the inlet end of the CEP, the reduction in flow velocity from that attained at the inlet end to that at the outlet end is such that the state of the alloy changes from the molten state at the inlet end to a semi-solid or thixotropic state at the outlet end.
In its flow beyond the outlet end, and substantially throughout a die cavity with which the flow path communicates, the alloy most preferably is retained in the semi-solid or thixotropic state. With sufficiently rapid solidification of alloy in the die cavity, and back from the die cavity back to or into the CEP, a resultant casting produced is able to be characterised by a microstructure having fine, spheroidal or rounded primary particles of degenerate dendritic form in a matrix of secondary phase. With sufficiently rapid solidification back into the CEP, the alloy solidified in the CEP is able to have a similar, related microstructure, but with this exhibiting fine striations or banding extending transversely of the CEP, that is, transversely with respect to the direction of alloy flow through the CEP. The striations or banding are a reflection of intense pressure waves which are generated in the alloy in its flow through the CEP. Those pressure waves give rise to the formation of the degenerate dendritic primary particles in generating the change in state of the alloy from a molten to a semi-solid or thixotropic state. The intense pressure waves also cause alloy element separation on the basis of density, with this being made manifest by the striations or banding, but also by radial separation of elements in the primary particles such as in a somewhat decaying sinusoidal form.
The use of a CEP in the inventions of the above-identified patent applications gives rise to a number of highly practical benefits. A principal one of those benefits is the microstructure detailed above. The primary particles are able to be less than 40 μm in size, such as about 10 μm or less. This fine primary phase and the fine matrix of secondary phase contributes significantly to physical properties of castings, such as tensile properties, fracture toughness and hardness.
A further benefit from the use of a CEP in those inventions is that substantial cost savings are obtainable. The savings result in part from the tonnage of alloy cast, to achieve a given product weight, being substantially reduced relative to the tonnage of alloy cast for the same product weight by current practice. The runner systems of current practice are large relative to the metal flow systems of those inventions, such that the volume and hence weight of solidified metal in the feed systems used in current practice is large relative to the casting volume and weight, and thus necessitate a higher tonnage of alloy cast to achieve the same product weight. Additionally, the tonnage of alloy loss also is correspondingly reduced with the reduction in tonnage of alloy cast. Moreover, those inventions facilitate production of a given casting on a smaller machine relative to current practice. Also, for a given casting, the use of a CEP in those inventions gives rise to greater flexibility in choice of location of an inlet to a die cavity, relative to the limited choice in current practice.
In general, the CEP of the inventions of the above-mentioned patent applications increases the range of shapes and sizes of castings able to be produced. This applies where die cavity fill is by direct injection in which an inlet to a die cavity is at a location from which alloy flows outwardly to peripheral regions of the die cavity. Indeed, the use of a CEP increases the opportunity to employ direct injection for many castings. However, the increased range of shapes and sizes of castings also applies where die cavity fill is by indirect or edge feed in which an inlet to a die cavity is at a location from which alloy flows across the die cavity and then peripherally, or simply flows peripherally, to achieve die cavity fill.
There are circumstances in which, despite the benefits of using a CEP, difficulties can be encountered in obtaining optimum benefit of the inventions of the above-mentioned patent applications. These difficulties may be evident from a required microstructure not being attained fully throughout a casting, due for example to an insufficient back pressure to alloy flow, or insufficient cooling, resulting from the geometric form of the die cavity for some castings. Generally the difficulties are encountered with indirect or edge feeding arrangements in the production of castings which are small in size and/or are relatively thin or have relatively thin sections. With these castings, it is difficult to control alloy flow velocities within the die cavity and, due to this and the small die cavity volume, die cavity fill time tends to be very short. Also, while the small die cavity volume is small and results in relatively rapid alloy solidification within the die cavity, the relatively low ratio of that volume to the volume of alloy in the metal flow system tends to result in an insufficient rate of solidification back from the die cavity along the flow path of the flow system.